Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to produce patterns in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to electromagnet radiation. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus generally includes an illumination system, a mask alignment stage for aligning the mask, a projection objective (sometimes also referred to as ‘the lens’) and a wafer alignment stage for aligning the wafer coated with the photoresist. The projection objective images, usually at a reduced scale, the circuit pattern, which is contained in the mask and illuminated by the illumination system, on the photoresist.
One of the main aims in the development of projection exposure apparatus is to be able to lithographically define features with smaller and smaller dimensions on the wafer. Small features lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components that are produced with the aid of such apparatus.
The minimum size of the features that can be lithographically defined is approximately proportional to the wavelength of the projection light. Therefore the manufacturers of such apparatus strive to use projection light having shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm and 157 nm and thus lie in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) spectral range.
The next generation of commercially available apparatus will use projection light having an even shorter wavelength of about 13.4 nm which is in the extreme ultraviolet (EUV) spectral range. Because there are no optical materials available that are transparent for EUV light, it is generally not possible to use lenses or other refractive optical elements in such an apparatus. Instead, the optical systems of such an apparatus are catoptric, which means that all optical elements (including the mask) have to be reflective.
The illumination system of an EUV projection exposure apparatus usually includes one or more multi facet mirrors. Sometimes one multi facet mirror is provided that is used to determine the intensity distribution in a pupil surface of the illumination system. This intensity distribution, in turn, determines from which direction EUV projection light impinges on the mask. Another multi facet mirror is used to produce a plurality of secondary light sources that commonly illuminate the mask. In principle, a multi facet mirror may also be used in the projection objective, for example at a position in or in close proximity to a pupil surface.
A multi facet mirror usually contains a support plate to which the individual mirror facets are attached. Depending on the application of the multi facet mirror, the individual mirror facets may be first adjusted on the support plate and then fixedly attached to it, for example by soldering. Other multi facet mirrors include mirror facets which can be adjusted at any time with the help of actuators. Such a multi facet mirror is described in US 2005/0030653 A1.
However, multi facet mirrors are not only used in EUV projection exposure apparatus, but also in the illumination systems of DUV or VUV apparatus. In such systems the multi facet mirrors can be freely tilted about two orthogonal tilt axes and are used to define the intensity distribution in a pupil surface of the illumination system. Several embodiments of such a multi facet mirror are described in WO 2005/026843 A1.
In a microlithographic projection exposure apparatus there is sometimes a desire to slightly modify the optical properties of the multi facet mirror. For example, an optical system of the apparatus may suffer from reversible or irreversible degradations which often compromise the overall performance of the apparatus. In such cases it would be desirable to be able to modify the optical properties of the multi facet mirror so that a corrective effect is achieved.
The optical properties of multi facet mirror are determined by the optical properties of the mirror facets, and these, in turn, are mainly determined by their curvature, orientation and position.
For changing the orientation of the mirror facets it has been proposed to provide each mirror facet with one or more actuators so that the mirror facets can be moved, and in particular tilted, individually. However, providing all mirror facets with actuators is costly and increases the system complexity considerably. Even if the mirror facets are provided with such actuators anyway, it is sometimes very difficult to produce with these actuators minute—but extremely accurate—changes of the orientation of the mirror facets.
Thus there is a desire to provide an optical system of a microlithographic projection exposure apparatus including a multi facet mirror that can be used for correcting optical aberrations.
Outside the field of microlithography multi facet mirrors are sometimes used that have individual mirror facets which are either in an “on” or an “off” state. Such multi facet mirrors are often referred to as digital micro-mirror device (DMD) and may be realized as micro-electric mechanical system (MEMS). For such a DMD it has been proposed in US 2007/0297042 A1 to attach the individual mirror facets on a common thin support membrane that rests on a plurality of posts arranged between the mirror facets. Groups of four posts define the corners of rectangular membrane portions, and to each membrane portion one mirror facet is attached. By deforming the membrane portions with the help of actuators it is possible to displace the mirror facets attached to it. The actuators are configured in such a manner that the membrane has only two stable configurations which correspond to the “on” and “off” state of the attached mirror facet.
A similar DMD, but with individual membranes for each mirror facet, is described in WO 2009/060906 A1.
In optical systems of microlithographic projection exposure apparatus DMDs usually cannot be used, because there is no need for a digital switching of light beams. Apart from that such apparatus have extremely tight specifications with regard to the optical properties (curvature, position, orientation) of the mirror facets, and these specifications are usually not met by DMDs.